The invention relates to a multi-targeting method for measuring distance according to the phase measuring principle and a computer program product.
In the area of non contact distance measurement, various measuring principles and measuring methods are known, which are described, for example, in the text books “J. M. Rüeger: Electronic Distance Measurement, 4th Edition; Springer, Berlin, 1996” and “R. Joeckel & M. Stober: Elektronische Entfernungs—und Richtungsmessung [Electronic distance and direction measurement] 4th Edition; 4. Auflage; Verlag Konrad Wittwer, Stuttgart, 1999”. Commercially available electrooptical distance measuring devices operate chiefly according to the phase measuring principle or the pulse transit time measurement principle; cf. for example Joeckel & Stober, chapter 13.
The mode of operation of these devices consists in transmitting modulated electromagnetic radiation, for example intensity-modulated light, to the targets to be surveyed and then receiving one or more echoes from the back-scattering objects, which are ideally exclusively the targets to be surveyed. The signal evaluation of the received echo is a standard technical task for which a multiplicity of solutions, in particular with the use of optical systems, was realised. In geodesy and the construction industry, tacheometers which are equipped with rangefinders measuring without reflectors have increasingly become established in recent years.
Distance measurement without reflectors often leads to situations in which the measured beam emitted by the rangefinder simultaneously impinges on a plurality of objects. This happens, for example, during the surveying of an edge; if it is measured, a part of the beam strikes the object with the edge while another part of the beam illuminates an object behind or the floor. A further example is a retroreflector which is present in the vicinity of a weakly reflecting target object and passes scattered light into the receiver of the rangefinder. A similar situation occurs if, unintentionally and often also unnoticed, the beam impinges on objects between the actual object to be measured and the instrument, for example in the case of distance measurements through window panes, tree branches, wire fences or wire grids.
In such multi-targeting situations, a conventional phase meter which outputs a single distance generally gives a false measurement, i.e. a measured distance value which contains an error which is far outside the specified accuracy of measurement. Transit-time meters can more easily recognise and handle multi-targeting situations provided that the targets are so far apart and the transmitted pulses are of a sufficiently short time that the echoes thereof can be detected and kept apart. In addition, transit-time meters have a larger range since their pulses can have a higher intensity than the continuously transmitted signals of the phase meters, without infringing eye safety regulations.
In spite of these two advantages of transit time meters, most customary tacheometers are equipped with phase meters because only in this way can they achieve the required accuracy of distance measurement in the mm or even sub-mm range with an effort suitable for field applications. The reliability of these devices would be substantially increased if their phase meters were to have multi-targeting capabilities.
WO 2004/074773 or EP 1 450 128 discloses a hybrid system for deriving geodetic distance information, in which a light signal is transmitted to one or more targets. Device components, such as transmitter and receiver, together with the targets are modelled as a linear time-invariant system which is activated by a signal and the system response of which is recorded. In contrast to pure transit time or phase meters, the distance information is derived both from the displacement as a function of time and from the signal shape of the system response.
Thus, the multi-targeting for phase meters which has not been technically realised or has been only with considerable complexity in hybrid systems proves to be a substantial disadvantage of all distance measuring principles known to date, once again only phase meters providing, with acceptable effort, the accuracy required for many applications. A main reason for this situation is the view, widespread among those skilled in the art and explicitly expressed, for example, in EP 1 450 128, that pure or exclusive phase meters, i.e. those which use no time signals, do not in principle have multi-targeting capabilities.